Katherine Grant, Claudette Colbert, is Mirror magazine’s top photographer, assigned to document the Interborough Vehicular River Tunnel project in New York City. Dressed in a hardhat and sturdy boots, she ventures underground to a bustling construction site where the tunnel workers—staunchly superstitious sandhogs—grumble about the bad luck a woman in the tunnel might bring. When she edges closer to the drilling point for a dramatic angle, her presence distracts the crew and triggers an accident that nearly crushes Jim Ryan, Fred MacMurray, the confident, muscular sandhog the others nickname “Superman.” He is knocked unconscious, and Katherine acts quickly to pull him to safety.
Inside the compression chamber, Ryan revives and clashes with his fellow sandhogs after they taunt him for being saved by a woman. Katherine captures the moment on film as Ryan stands up to the taunts, revealing a raw, unpolished bravery that fascinates her even as it unsettles him. On their way out, Ryan notices Katherine’s flirtatious glance and bluntly tells her he’s not interested; she fires back that she has a boudoir chair with more integrity than he does.
Back at home, Katherine shows her photographs to her sister Hoppy, Ilka Chase, and their circle of refined friends—Henry Fulton, Paul McGrath, the Mirror publisher; Dunbar, Morton Lowry, the playwright; and Roger Winant, Richard Haydn—who mock the rough-edged “ape” she has captured. When Ryan arrives to retrieve the tripod she left behind, he confronts the mockery with a quick reenactment of the brawl, roughing up the mockers and proving he’s no simple brute. Katherine’s daydreams drift into fantasies where Ryan becomes the heroic savior she imagines, blurring the line between work and wishful thinking.
In the weeks that follow, Henry publishes the photograph of the sandhogs fighting, and Ryan loses his job as a result. Seeing an opening, Katherine offers him a job as her assistant to help replace his lost wages. Despite his flirtations with several cheap blondes, Katherine finds herself drawn to him, and they gradually fall into a tentative romance. After a shared kiss, Katherine’s prudish social world catches up with them when Hoppy reveals Katherine’s original plan to push Ryan out of her mind. Feeling used, Ryan walks away in disappointment.
The tunnel project is then threatened by repeated cave-ins, caused by the constant muck flooding from the riverbed. The only way to stabilize things is a new machine and chemical process developed by Ryan—an engineer who has been working as a sandhog to gain practical insight into underwater construction. Katherine returns to the tunnel to cover the tests and discovers that the inventor behind the improvements is Ryan. She secures a vantage point to photograph the crucial test as the muck begins to solidify. A sudden breach sends a muck slide toward her, but Ryan and the crew risk their lives to pull her to safety. The officials declare the test a partial victory, not yet conclusive.
Determined to prove the machine’s worth, Katherine exhorts the officials to review the proof she captured—the photos show the muck flow abating when the breach happened—and presents them with the evidence, along with her condition that Ryan must never know she was the catalyst behind the decision. With this concession, she accepts Henry’s marriage proposal, hoping to navigate a path that keeps her professional integrity intact even as her heart leans toward Ryan.
Meanwhile, Roger visits Ryan to reveal Katherine’s plan to marry Henry, unaware of her true feelings for Ryan. Darlene, one of Ryan’s showgirl acquaintances, overhears and storms into Katherine’s engagement party to warn her to stay away from Ryan. The confrontation escalates when Katherine throws a punch in the heat of the moment, and Ryan arrives to witness the stand-off. He retrieves Katherine, lifting the skeptical Henry with one hand and holding the “chair” in the other, offering Katherine a final, personal choice. She chooses Ryan, and the couple walks off together, ready to face whatever the future holds in a city that never stops building.
